\begin{tikzpicture}
    \tkzDefPoints{0/0/O, 1.5/0/A}

    \tkzDrawCircle[thick](O,A)
    \tkzDrawSegment(O,A)
    \tkzDrawPoint(O)
    \tkzLabelSegment[above](O,A){$r$}
    \tkzLabelPoints[left](O)
    \tkzLabelPoints[right](A)
\end{tikzpicture}

